1. Field of the Invention
The present invention relates essentially to a process for the preparation of microorganisms enclosed in appreciably dehydrated gels, gels obtained and their use for the preparation of fermented drinks.
2. Description of the Related Art
In the state of the prior art, it is known that immobilized microorganisms can be used in the production of fermented drinks such as wine and beer (FR-A-2 320 349 and FR-A-2 359 202). Their use has also been suggested for classical champagnization (FR-A-2 432 045) as well as for the manufacture of sparkling drinks with a variable degree of alcohol (FR-A-2 601 687). These publications, as well as others (in particular JP-A-57-150 385 or EP-A-173 915) have emphasized the performances of reactors with immobilized cells.
These techniques have made possible the carrying out of fermentations with mixtures of microorganisms of different categories (mixture of lactic acid bacteria and mixture of yeast).
However, the implementation of the process on an industrial scale came up against the difficulty of having available particles capable of being preserved for a long period.
Microorganisms in the enclosed form may be used without an appreciable drop of activity over long periods when the nutrition of the micro-organisms concerned is respected.
It has been possible to note a toxicity of the products of fermentation which leads to a partial ageing of the cells (article by DIVIES et al. in Annales de microbiologie, 1977, pages 349-358).
It was thus preferred in this particular case to use a battery of reactors of defined life time and to carry out the partial renewal of the microbial particles in a programmed manner.
Problems of mechanical resistance of the gel have also managed to arise and are described in JP-A-57-150 385.
It thus appears crucial at the industrial level to plan the elaboration of the particles for inclusion of the microorganisms and to centralize the production of them. It is also necessary to bring about the inclusion of the microorganisms in the gels which ensures them excellent viability over a period of time.
Now, it became apparent that the microorganisms enclosed in gels were very sensitive and tolerated storage for a prolonged period of time with difficulty.
In order to overcome this disadvantage of storage, storage processes including a drying have already been suggested.
The commercial preparations of microorganisms enclosed in dried gels naturally need to be capable of rehydration and maintain excellent viability after rehydration.
Certain solutions have been proposed which permit storage over periods which may attain 6 months to 1 year in a protective packing at a relatively low temperature of 4.degree. to 10.degree. C. (see BECKER and RAPOPORT in Advances in Biochemical Engineering and Biotechnology, Volume 35, 1987, pages 128 to 171).
The TATE document EP-A-O 065 376 describes a process for the preparation of enzymes immobilized in a gel which is then dried if appropriate and, after drying, is placed in contact with glycerol (page 10, 2nd paragraph and claim 3). A placing in contact after drying does not make it possible to preserve the structure of the gel which renders its rehydration difficult, a problem which is resolved by the present invention which will be described below.
The document FR-A-2 519 022 describes a process for the preparation of inocula with long viability and having an improved resistance to temperature which comprises a drying of the microorganisms in accordance with various drying processes. This document also describes in its introduction many documents of the prior art relating to drying of gels enclosing microorganisms.
According to this document FR-A-2 519 022, at the start a culture of microorganisms is grown in a standard culture medium for several days.
To this culture medium, a gellable polymer, which may be a polysaccharide, xanthan or an alginate, may be added.
After gelation which enables the inclusion of a microorganism in the culture medium to be carried out, drying is undertaken until an activity of water in the inoculum is produced below its critical value to a value lower than 0.5, this value being maintained during storage (see claim 1, in particular). Preferably, the activity of water in the inoculum is maintained below 0.3 and preferably even below 0.1 (claim 2).
It is to be noted that according to this document attention is not paid to the special problem of the rehydration of the dry or appreciably dehydrated gel so as to obtain rehydrated gels having a structure approximately identical with that which they possessed before their dehydration.
Now, experience has shown that the dehydrated or dried particles obtained by the procedure described in this document rehydrate with very great difficulty. In the best cases, in the presence of culture medium, the rehydration remains limited at 20% humidity, the particles always remaining very small, dried up, ungraded with respect to size and very hard.
If a supplementation with hydrophilic substances such as carrageenin or carob seed grain is carried out, this causes the gel to become brittle without improving the rehydration of the gelled polymer, and this is particularly true in the case of the employment of an alginate. In the best cases, viabilities incompatible by some few percent with an industrial use may be obtained.
Hence, it appears necessary to dispose of easily rehydratable gels containing a maximum of viable microorganisms after rehydration, and to do so even after a long period of storage.